Traditional polymerizable dental materials, as are for example described in EP-0 091 990 A2, in most cases contain crosslinked bi- or polyfunctional acrylates and methacrylates which are predominantly radically polymerized. It is a disadvantage that in the case of all the polymerizable dental materials known to date, polymerization is associated with a clear decrease in volume. The shrinkage of customary monomer mixtures is in the range from 5 to 12% by vol. In the case of filled composite materials, the decrease in volume is in the range from 2.6 to 7.1% by vol. (A. J. Feilzer, A. J. DeGee, C. L. Davidson, J. Prosthet. Dent. 59 (1988) 297).
The reduction in volume means that no adequate, load-resistant marginal adaption can be achieved, particularly in the side tooth region (I. Kerjci, Zahnfarbene Restaurationen, Hanser Verlag Munich/Vienna, 1992, page 5 et seq.). In the case of poor marginal edge adaption, there is the danger, particularly in regions which are only poorly accessible by dental hygiene measures, that bacteria will find their way between tooth and filling and thus damage the pulp or trigger the formation of secondary caries. Moreover, a reduction in volume upon polymerization has a negative effect on the mechanical properties of the material.
Although dental material shrinkage upon polymerization can be reduced by using monomers with higher molecular weights whilst simultaneously lowering the percentage proportion of the polymerizable group relative to the molecular weight of the molecule, the molecular weight increase brings about a considerable, undesired increase in the viscosity of the dental material, which makes its further processing, such as for example the incorporation of fillings, considerably more difficult.
The polymerization of liquid crystalline (LC) monomers produces so-called side-chain liquid crystalline polymers (SCLCP). These are suitable primarily for reversible information storage, the production of media with non-linear optical properties, for the production of optoelectronic construction elements, as separating phases for chromatographic procedures and as coating materials (J. Rubner, R. Ruhmann, G. Rodekirch, Plaste und Kautschuk 36 (1989) 253).
In most cases, polymerizable liquid crystalline monomers contain styrene or (meth)acrylate groups as polymerizable groups, whilst their mesogenic groups are frequently derived from aromatic carboxylic acid esters, azomethines or steroids (A. Blumenstein, Liquid Cristalline Order in Polymers, Academic Press, New York, 1978, page 105; J. H. Wendorff, Flussigkristalline Polymere, C. Hanser Verlag, Munich/Vienna, 1989). Liquid crystalline monomers can for example be polymerized by light (D. J. Broer, K. Katsumi, Makromol. Chem. 189 (1988) 185), ionically, such as for example in the case of the cationic polymerization of liquid crystalline vinyl ethers (H. Jonson, H. Andersson, P. E. Sundell, U. W. Gudde, A. Hult, Polym. Bull. 25 (1991) 641) and by group transfer polymerization, such as for example in the case of liquid crystalline methacrylates (W. Kreuder, O. W. Webster, H. Ringsdorf, Makromol. Chem., Rapid Commun. 7 (1986) 5).
In addition to liquid crystalline compounds with a polymerizable group in the molecule, various difunctional liquid crystalline monomers are known, for example diacrylates (S. C. Lin, E. M. Pearce, High-Performance Thermosets, Hanser Pub. Munich, Vienna, New York, 1993, page 270), divinyl ethers (H. Andersson, F. Sahlen, U. W. Gedde, A. Hult, Macromol. Symp. 77 (1994) 339) or diepoxides (S. Jahromi, J. Lub, G. N. Mol, Polymer 35 (1994) 622). The polymerization of difunctional liquid crystalline monomers produces ordered network polymers.
The use of liquid crystalline monomers for the production of dental materials has not been described to date.